The fall started when Cas told Dean he loved him. That was the breaking point in heaven. An angel had never loved a human before. Not like this. It was beyond Naomi's control, and she lost him. He slipped right between her fingers and into Dean Winchester's arms.
And Dean loved him back. Maybe he didn't light candles and spread rose petals but he let him borrow his clothes. And he got him his very own shotgun. Sam knew about them, and he was happy. Maybe now that they both had a bit more to cling to here on earth, they would decide to stay.
The fall was slow and painful for Castiel. He couldn't fly anymore, and what little healing power he kept he had to use on himself. For an angel, he sure was a clumsy bastard. Scraped knees, skinned hands, he was always getting hurt. One time he broke his arm so bad they had to put him on meds.
He didn't get to fight in the battle in Detroit, so he didn't see Sam say yes. But he saw Dean's face when he got back and he knew. He could tell by the emptiness and the cold. He took a percocet, which kind of helped rid the pain of watching Dean fall apart. But not really.
Eventually, he became reliant on the pills. They kept him sane in a world where he could think now. He could think for himself, and sometimes he thought so hard it hurt. He knew too much to be human, now, and the knowledge pressed out against his skull until he downed another two white pills labeled 512.
By the time he'd been mortal for over a year, he was strung out on 5 different kinds of shit. Dean supplied him, mostly to keep away the nightmares of what he liked to call Withdrawal Cas. It pained him to hear the click of pill bottles every 4 hours but it helped Cas and that was what was important, right?
Soon he got into pot ("dammit Lou, I told you to keep him outta that shit!") and then after a while, coke. Fucking Roy introduced him to tattoos, and thus needles. Dean forbade needles. The worst drug came in a syringe, all nice and clinical, and it was a killer. THE killer.
The night Dean found him shooting up was the night he died. Dean cried and screamed at him, pulling the needle out of his arm and throwing it across the room, where it clattered to the floor, seemingly innocent. And he held his head up as he tried to puke, nothing coming up but bile. He wiped the tears from Cas' cold cheeks as the light went out of his eyes. And he cried his own as he cradled his lover in his lap, losing the one last person that gave a damn about him.